The present invention relates to an improved wire feed system for electrical discharge machining which in many cases eliminates the necessity of relatively expensive three dimensional electrical discharge machining electrodes that must be machined by precision machining operations which greatly add to the cost of the operation. The present invention by its use of a precisely driven and guided wire electrode makes possible the generation of very small radii in intricate shapes. As the wire electrode process has expanded to press tooling, extrusion dies, powdered metal dies and precision prototype parts, it became necessary to improve the machine tool head and to provide a wire feed system design which is specifically made for wire feed and operable to accommodate the necessary standards of reliability and performance. It has been necessary to completely redesign the head assembly of the standard EDM machine tool to provide for the wire feed system.
It will be understood that the present invention as it relates to the machine tool portion of the EDM apparatus is used in conjunction with an EDM power supply which comprises a pulse generator that provides machining power pulses to the machining gap. The machining power pulses are of precisely controllable on-off time and current magnitude therefore to control the rate of machining, the finish of the workpiece and the degree of the overcut.
In the use of a wire electrode, the level at which machining, is accomplished is typically of a high voltage level, which may range as much as 300 volts. A typical range of machining current would be 15 amperes of cutting current either infinitely variable or with as many as 10 steps of current limiting provided.
It will be understood that the tables on a typical two axis contouring model, such as is shown in the drawings, would be capable of performing both straight line motions and continuous path contouring and any of a number of commercial numerical tape control units are available for providing this control. One example of such a system is the numerical tape control unit manufactured by the Superior Electric Company of Bristol, Conn.
The prior art has shown a variety of wire and band electrode arrangements which are adapted to provide the type of machining with which we are currently concerned. However, certain difficulties have arisen in the use of such systems. Among the major problems are the requirement for causing a length of wire to pass with a constant velocity through the machining gap region. This constant velocity movement is necessary since the cross section of the wire changes during machining. Unless the movement of the wire through the gap is maintained at a constant velocity, the machining slot would be of a varying width. This condition could not be tolerated in any precision machining operation.
An example of an early type of band electrode machining arrangement is shown and described in U.S. Pat. No. 2,903,557 issued on Sept. 8, 1959 to V. E. Matulaitis for "Arc Machining With Band Electrode." In that patent, the problem of maintaining a constant width band or wire electrode was handled by providing a feed of the electrode with a velocity that was regulated in accordance with a feed-back signal representative of average current through the gap. The system for guiding and driving the band electrode consisted of a pair of reels, that is, a supply reel and a windup reel. In the two reel system, it was exceedingly difficult to avoid electrode bend in the wire between the two reels.
Other prior art arrangements are shown and described in U.S. Pat. No. 3,731,043 issued on May 1, 1973 to Ullmann et al. for "Digital Circuit for an Eroding Machine" and U.S. Pat. No. 3,636,296 issued on Jan. 18, 1972 to G. G. Semin for "Arrangement for Stopping Automatically an Electric Spark Eroding Machine Having Its Work-Performing Electrode Tool in the Shape of a Wire or Band." Both of these patents show systems similar to the Matulaitis patent in that the wire or band electrode at its ends is supported and unreeled from spools and reeled onto spools. Such an arrangement has in it the difficulty of providing frequent and unpredictable bends with accompanying changes in velocity of the electrode band or wire such that the gap being machined varies in thickness. Accordingly, the overcut and the dimensions of the machined article cannot be maintained within the necessary limits.
An additional shortcoming not overcome by the prior art is the problem of maintaining that section of the wire electrode which is doing the cutting, that is in the region proximate to the workpiece, in a fixed, properly oriented path to achieve the required cut. Typically, if the control exercised is a two dimensional control with an X- and a Y- axis table, it would be necessary to maintain the wire with its operative cutting length perpendicular to the table carrying the workpiece or, alternately stated, perpendicular to the plane in which the workpiece is lying.
The invention of applicant with respect to the alignment and support of the wire in this critical region includes a novel and highly effective arrangement between two orthogonally arrayed guide rollers, adjustable along their axes of rotation, such that this precise alignment is not only obtainable from the very beginning of the cutting operation but is continued throughout the cutting cycle. As shown in the aforementioned U.S. Pat. No. 3,731,043, the wire in the critical cutting region is maintained between simple pins. In the aforementioned U.S. Pat. No. 3,636,926, the wire is supported between parallel axis rollers so that the major benefits of a system like applicant's are not possible.